A conventional control apparatus is described in U.S. Pat. No. 5,481,168. In this conventional apparatus, the rotation speed of a three-phase induction motor is delivered via an analog/digital converter to a rotation speed measurement device in order to regulate an asynchronous machine. A setpoint transducer predefines a torque, and these signals are fed into a three-phase system to generate setpoint values. The sensed rotation speed of the asynchronous machine is compared to the predefined rotation speed, and the difference is delivered to the power section of an inverter as the setpoint value.
Another conventional control apparatus of this kind is the so-called "field-oriented" control system as described, for example, in Siemens Research and Development Reports 1972, F. Blaschke, "The field orientation method for controlling asynchronous machines", p. 184 ff., or in the textbook "Control of Electrical Drives," W. Leonhard, Springer-Verlag, pp. 214-222. With the field-oriented control system, both the amplitude of the stator flux vector and its position with respect to the rotor flux vector must be monitored at all times. One of the principal tasks in this connection is decoupling the torque-based and flux-based currents from the magnitude of the stator current vector. It is also important to ensure that they are at right angles to one another, in a rotor-based coordinate system, at all times. This requires sensing the stator currents of the three-phase system, transforming them into a coordinate system which rotates synchronously with the rotor flux, and comparing them with the setpoint definitions for the flux-forming component and torque-forming component of the current. The new current/voltage values applied to the motor are based on calculation and inverse transformation from the rotating reference system to the steady-state stator coordinate system. The field-oriented control system yields a constant torque even above the nominal rotation speed, improved velocity consistency even under fluctuating load conditions, and high efficiency at full load, but the technical outlay is high and costly.